1. Field of the Invention
The present invention relates to an image forming apparatus of, for instance, a copying machine or a laser beam printer, etc. which is provided with a plurality of developers.
2. Description of the Prior Art
General speaking, a developer for each of the colors is placed in the developer system installed in a color image forming apparatus of, for instance, a copying machine or a laser beam printer, etc. Various kinds of systems have been adopted conventionally as to configuration of arrangement of these developers. Such configurations are usually divided into two large groups, one of which is a fixed developer system and the other of which is a movable developer system.
In the fixed developer system, as developers corresponding to each of the colors are fixed, a moving mechanism for these developers is not needed. However, it is necessary to increase the drum diameter of a photosensitive drum in order to secure a large enough space for arranging the developers in the vicinity of the photosensitive drum. Also, as the developing position opposite to the photosensitive drum differs according to each of the colors, the composition of these developers is subjected to some limitation, and there is a disadvantage that common use of these developers can not be attempted to be accomplished.
On the other hand, in a movable developer system in which each of the developers is subjected to make rotary movements or linear movements, it is possible to adopt a photosensitive drum having a comparatively small diameter as well as for black and white development, and there is and advantage that common use of the developers can be accomplished in each of the colors.
According to the above reasons, recently, a movable developer system has been often adopted. As one of the conventional examples of such a movable developer system as explained in the above, such a configuration, in which each of the developers is integrally linearly moved, so as to be opposed to the photosensitive drum is shown in FIG. 7. A developer device 60 of a color copying machine shown here in this FIG. 7 is such a type as can duplicate with four full colors, in which a magenta-color developer 61, a cyan-color developer 62, a yellow-color developer 63 and a black developer 64 are longitudinally placed and arranged from the above in this order. As these four developers 61 through 64 are integrally driven vertically, each of the developers is changed by turns, opposed to the photosensitive body of photosensitive drum 65, thereby causing an electrostatic latent image on the photosensitive drum 65 to be developed.
The developers are so composed that the position thereof can be compensated at an appointed compensating position on the way of a movement so as to be accurately opposed to the photosensitive drum 65, thereby causing such an abnormality as "imbalance or unevenness of the developing density" of an image obtained by the above development processing to be prevented.
In the conventional image forming apparatus, a usual AC motor or DC motor has been used as motor of the moving mechanism for moving the developers. And only in the case that only such a motor as described in the above is used, the positioning accuracy pertaining to the developers is not good. Hence, in order to heighten the positioning accuracy, a mechanical stopper mechanism is further added.
In the case that such a mechanical stopper mechanism is not added, it can be considered that in order to make the position compensating more accurate a developer is changed after the same developer moves to a compensating position and the position thereof is compensated whenever changing one developer to the other one.
In an image forming apparatus furnished with such a conventional mechanical stopper mechanism as explained in the above, as a stop mechanism is needed in addition to the motor, the cost of production is increased. Additionally, as the positioning of the developer relies upon a mechanical braking force, the positioning accuracy pertaining to the developers will be worsened as the moving speed of the developers is accelerated.
In a method in which position compensating is carried out every time when one of the developers is changed to the other, unnecessary time for moving to the compensating the position of the developers is needed. Therefore, in the conventional image forming apparatus, it is difficult to use a method for compensating the position of developers whenever changing the developers as explained in the above.
On the other hand, in the case that continuous copying of four full colors is carried out in a developer unit of such a linear moving type as mentioned above, development is carried out by changing the developing colors in an appointed cycle of magenta-color.fwdarw.cyan-color.fwdarw.yellow-color.fwdarw.black.fwdarw.ma genta-color.fwdarw. . . . as the characteristics of the constituents of the developing agents is taken into consideration. In the case that such reciprocatory movements of the developers mentioned above are performed, the opposed position of the photosensitive drum must move from black to magenta-color, leaping over cyan-color and yellow-color, and the moving distance in this case will become equivalent to three times the pitch of the developers (three pitches), wherein one pitch is equal to the distance between two neighboring developers.
Hence, in the case that the reciprocatory moving system is employed, the time necessary for changing from a certain color developer to the next color developer must be set to the same period of time in order to facilitate the control system thereof. Therefore, in order to make the period of the moving time required for the three pitches of the developers almost same as that of the moving time required for another pitch, a motor of large capacity will be needed so that it can rotate at a high speed. On the other hand, in the case that the period of the moving time for another pitch is set in accordance with the period of the moving time for three pitches of the developers by using a small-sized motor, the total period of the moving time will be lengthened, thereby causing high speed development to become impossible.
On the other hand, there is another system in which a plurality of developers is moved rotatably, as another example of the moving type developer system. In this developer system, even when the developing color is changed from black to magenta-color as shown in the above, the period of the moving time is enough with one pitch, and such a difficulty as mentioned above does not result therefrom. But in the case that the rotary moving system is adopted, it becomes difficult to supplement toner to the developers which are in each of the positions thereof, and as developers of rotary moving system rotates endlessly, the contacts of the electric system thereof become very complicated. Also, a developer may be turned upside down. Therefore, another difficulty for taking a countermeasure thereagainst results therefrom.